This invention relates to a speed responsive engagement device, and in particularly to a speed responsive engagement device for use in fall arrest apparatus.
Speed responsive engagement devices for use with rotating parts to selectively engage the rotating parts to other components if the speed of rotation of the parts exceeds a threshold value are well known.
There are two main types of such speed responsive engagement devices. A first type of speed responsive engagement device are devices of the centrifugal clutch type. A schematic view of such a centrifugal clutch is shown in FIG. 1. The centrifugal clutch has pawls 1 rotating within a fixed circular ratchet 3 and mounted on a rotating component 2. The pawls 1 are arranged for pivotal movement so that they can move between an inner disengaged position where the pawls 1 are not engaged with the ratchet 3 and the component 2 can rotate freely, and an outer engaged position where the pawls 1 are engaged with the ratchet 3 so that rotation of the component 2 relative to the ratchet 3 is prevented. The pawls 1 are biased inwardly towards the unengaged position, generally by resilient means such as springs 4, and are arranged to be biased outwardly by centrifugal forces as the component 2 rotates. By appropriate selection of dimensions and components the centrifugal clutch can be arranged to engage the pawls 1 to the ratchet 3 at a desired rotational speed of the component 2.
A second type of speed responsive engagement device are devices of the rocking pawl type. A schematic view of such a rocking pawl device is shown in FIGS. 2A and 2B. The rocking pawl device has a ratchet mechanism in the form of an outwardly facing ratchet wheel 11 attached to the rotating component 12 and a pawl 13 contacting the ratchet wheel 11. The pawl 13 is able to pivot between a first unengaged position shown in FIG. 2a in which the pawl 13 is not engaged with the ratchet wheel 11 so that the component 12 is able to rotate freely, and a second engaged position shown in FIG. 2b where the pawl 13 is engaged to the ratchet wheel 11 preventing rotation of the component 12 relative to the structure to which the pawl 13 is attached. The pawl 13 is biased by resilient means 14 towards the first unengaged position. As the component 12 rotates and the ratchet wheel 11 rotates relative to the pawl 13, each tooth of the ratchet wheel 11 in turn strikes a first end of the pawl 13 and pushes it outwardly away from the ratchet wheel 11 so that the pawl 13 pivots towards the engaged position. The pawl 13 is resiliently biased towards the first unengaged position, and as a result this outward pushing of the pawl 13 by the ratchet wheel teeth results in an oscillating movement of the pawl 13 from the unengaged position towards the engaged position and back towards the unengaged position. As the speed of rotation of the component 12 and the ratchet wheel 11 increases the amplitude of the oscillation of the pawl 13 increases until the amplitude of the oscillation is large enough to bring the pawl 13 into the engaged position. The pawl 13 then engages with the ratchet wheel 11 and rotation of the component 12 relative to the pawl 13 and structure to which is attached is prevented. Again, the dimensions of the components can be selected to set the threshold rotational speed at which the pawl 13 is driven into engagement with the ratchet wheel 11 to a desired value.
The speed sensitive engagement means of both of these known types are used in fall arrest systems of the type where personnel working at height are attached to a safety line wound around a drum. The drum has an automatic rewinding mechanism and a speed sensitive attachment mechanism of the type disclosed above, which responds to the rotation of the drum at a speed above a predetermined threshold by locking the drum against further rotation relative to the drum support or other fixed structure.
In use, fall arrest systems of this type allow personnel to move freely around a working area including moving upwardly and downwardly in the area, with the safety line being automatically paid out from and wound onto the drum under the control of the automatic rewinding mechanism as necessary to allow the personnel movement and keep the safety line taut. When a personnel fall occurs, the safety line is pulled out at a much greater speed then is necessary for normal movement and the speed of rotation of the drum rises to the threshold value of the speed sensitive engagement means, which locks the drum against further rotation and so arrests the fall.
In practice it has been found that there is a problem with fall arrest systems of both of these known types that after a fall has been arrested the speed sensitive engagement means can release the engagement allowing the personnel to again start to fall until the speed of rotation of the drum again reaches the threshold value and the fall is again arrested.
It has been found that it is possible for this cycle to be repeated so that personnel drop in a series of short falls until they reach the ground or some other supporting surface.
This problem is generally referred to as bounce.
Bounce is dangerous and presents a serious problem for a number of reasons. Firstly, the personnel may be injured by impact with other objects during the multiple falls. Further, in general fall arrest systems are designed so that users undergoing a fall arrest event are only subject to a safe level of force. However, these safe levels are calculated on the assumption of single fall event. Even when a single application of a fall arrest force is safe, repeated application of the same force to a user can result in injury. This problem is made more severe by the fact that many fall arrest systems include single use energy absorbing or shock limiting devices so that successive falls and arrests resulting from bounce may result in personnel being subject to higher than expected levels of force because the capacity of the single use energy absorbing or shock limiting devices in the system has been used up. Further, the repeated fall and arrest loads on the fall arrest system due to bounce can result in failure or damage of components of the fall arrest system or the supporting structure to which it is attached. Finally, where bounce results in personnel descending all the way to the ground or other supporting structure in a series of short falls the final impact with the ground or other support structure may be at a sufficiently high speed to cause injury.
The present invention was made in any attempt to provide a speed sensitive engagement device overcoming this problem.